In virtually all light sources such as tungsten halogen, incandescent, metal arc, etc., the electric energy is coupled to the lamp by means of conductors sealed into the glass envelope or bulb by means of a glass-to-metal seal. In the case of tungsten halogen lamps, the sealed end is typically press-sealed and is usually formed by having thin metal foil or wire pressed between the end's molten glass material. Because the metal and glass in contact with one another have different coefficients of expansion, they expand to different degrees upon heating. This expansion difference causes stress in the seal when heated and, generally, the greater the heating, the more stress that is imposed. Failure of a lamp seal in turn will cause failure of the lamp. The aforementioned heat not only causes the resulting stress but also causes accelerated oxidation of the parts (i.e., foils) exposed to air. Therefore, maintaining the press-seal at a cool temperature is a major consideration in the design and manufacture of lighting products such as these.
Most lamp seals are enclosed in a base that forms part of the lamp. The base not only protects the relatively sensitive seal area from physical damage, but also isolates the seal from the radiant energy given off by the lamp filament, or reflected from the lamp fixture.
There are primarily two types of lamp bases employed in tungsten halogen lamps. One such base is a formed metal base, as illustrated in U.S. Pat. No. 4,371,807. In addition, the assignee of this invention manufactures and sells single ended tungsten halogen lamps with metal bases having vent holes in the side walls of the metal base. The other form of base is a solid ceramic lamp base. Examples of tungsten halogen ceramic bases are illustrated in U.S. Pat. Nos. 4,243,907 and 4,568,854, both owned by the present assignee herein.
As indicated previously, the prior stamped metal base is provided with holes in the side wall, These bases are usually placed there using a punching operation. Such is necessary with a metal base, not only to help remove the heat added to the seal from conduction through the leads, but to cool the base itself. Because metal is an excellent heat conductor, radiant energy striking the base from the outside is immediately conducted to the inside of the base. The holes in the base help to minimize this heating effect. Because the metal itself is also sensitive to both heat and corrosion, the holes in the metal base also tend to cool the metal so as to protect the base itself.
Thus, there is a very definite advantage to having these holes for cooling purposes. However, there are also definite disadvantages. The holes in the base allow the passage of radiant energy to the seal area. Additionally, light and heat reflected from fixture optics and reflectors strike the seal and elevate its temperature. Further disadvantages associated with a metal base include the fact that these are relatively expensive and also that they tend to be sensitive to corrosion over time.
The solid ceramic base protects the seal from radiant heating by sealing it off. This ceramic base has proven to be a very good insulator and is generally white or light in color, therefor, allowing no radiant heating effect. However, by enclosing the seal the ceramic base forms a nearly stagnant air pocket about the seal which allows for very little cooling of conducted heat. The heat conducted down the electrical leads from the lamp filament or heat source is trapped and not allowed to escape.